Material, particularly for an optical component for use in microlithography, and method for making a blank from the material

ABSTRACT

The invention is concerned with a material which shows low absorption for UV radiation having a wavelength below 250 nm, low birefringence, high chemical resistance and high radiation resistance and which is therefore particularly usable for Making optical components for microlithography. According to the invention the material consists of synthetically produced quartz crystallites which form a polycrystalline structure and have a mean grain size in the range between 500 nm and 30 μm. The method according to the invention for making a blank from the material comprises providing granules consisting of synthetically produced quartz crystals having a mean grain size in the range between 500 nm and 30 μm, and sintering the granules to obtain a blank of polycrystalline quartz.

The present invention relates to a material, particularly for an opticalcomponent for use in microlithography, and to a method for making ablank from the material.

The optical components made from the material according to the presentinvention are generally used for the transmission of high-energyultraviolet radiation, especially in exposure and projection optics ofmicrolithography devices within the scope of making large-scaleintegrated circuits for semiconductor chips. The achievable resolutionof the exposure or projection objective depends on the operatingwavelength. At the moment microlithography devices are predominantlyequipped with excimer lasers which emit UV radiation of a wavelength of248 nm (KrF laser) or of 193 nm (ArF laser).

On account of its mechanical and chemical resistance and its lowbirefringence, quartz glass is a preferred material for makinghigh-quality optical components. In optical components made from quartzglass, short-wave UV radiation may however produce defects leading toabsorptions. Type and extent of a defect formation depend on the typeand quality of the respective quartz glass, which is essentiallydetermined by structural properties, such as density, refractive indexprofile, homogeneity and chemical composition. Therefore, with furtherincreasing demands made on radiation resistance or with a progressiveshortening of the operating wavelength, physical limits and limits dueto the material must also be expected in the case of quartz glass.

As an alternative to quartz glass, synthetically produced crystallinematerials that are transparent to short-wave UV radiation and aredistinguished by high UV radiation resistance are being tested as lensmaterial. Crystalline fluorides such as potassium fluoride (CaF₂) orbarium fluoride (BaF₂), crystalline alkaline earth oxides, particularlyMgO, as well as single crystalline or polycrystalline spinel (MgAl₂O₄)should here be mentioned as examples. However, it has been found thateven single crystals with cubic lattice structure at short-wave UVradiation show considerable intrinsic birefringence which significantlyimpairs the imaging fidelity of the optical component made therefrom.

However, many crystalline materials (such as CaF₂), which with respectto their UV radiation resistance and transparency would as such besuited for use in microlithography, show a poor chemical resistance.This is above all a drawback when the optical component made from thecrystal is in contact with a liquid, for instance in the case ofmicrolithographic projection systems operating according to thetechnique of the so-called “immersion lithography”. The gap between thelast optical component of the lens system and the substrate to beexposed is here filled with a liquid having a higher refractive indexthan air.

A further difficulty is due to the standard manufacture of artificialcrystals through a crucible melting method, known as the Bridgmanmethod, which may lead to an input of impurities out of the cruciblematerial into the crystal and may thereby produce absorption bands inthe range of the operating wavelength.

It is therefore the object of the present invention to provide amaterial for making an optical component, the material showing lowabsorption for UV radiation having a wavelength below 250 nm, lowbirefringence, high chemical resistance and high radiation resistance,and being therefore particularly usable for making optical componentsfor microlithography.

Furthermore, it is the object of the present invention to indicate amethod for making such a material.

As for the material, this object is achieved according to the inventionin that the material consists of synthetically produced quartzcrystallites which form a polycrystalline structure and have a meangrain size in the range between 500 nm and 30 μm.

The polycrystalline material according to the invention consists ofsynthetically produced quartz crystallites of a specific grain sizerange.

-   -   The synthetic production of the quartz crystallites permits a        high purity of the material and thus an adequate transparency in        the wavelength range below 250 nm up to about 150 nm.    -   The specific grain size of the quartz crystallites shows several        important aspects:        a) The light passage through the polycrystalline quartz        structure is reduced by diffuse scattering on pores and        additional light scattering by birefringence at each transition        of the light beam from one into the next crystallite of the        structure.

Normally, crystalline quartz is present in the trigonal α-quartz phasewhich is stable at normal pressure up to a temperature of 573° C. Inthis polycrystalline quartz material the scattering amount based onbirefringence is reduced by small structure grain sizes. This yields amaximum mean size of the quartz crystallites of 30 μm.

b) On the other hand, during light passage through the polycrystallinequartz structure a loss portion is also observed due to scattering ongrain boundaries. Therefore, light scattering increases with anincreasing number of grain limits and thus with an increasing number ofquartz crystallites per volume unit. This yields a minimal mean size ofthe quartz crystallites of 0.5 μm.

c) Moreover the effect of birefringence of each individual quartzcrystallite is eliminated by the statistic distribution of theorientation of the individual crystallites in the polycrystallinestructure, so that the optical component produced from the materialshows low birefringence of less than 1 nm/cm on the whole.

-   -   Moreover, what is particularly important is that, due to its        crystalline structure, the polycrystalline SiO₂ material has a        distinctly higher resistance to short-wave UV radiation as        compared with quartz glass.

For the determination of the mean grain size of the quartz crystallitesa standard software-controlled image evaluation is primarily possible oralso counting according to the so-called circle method.

The crystalline SiO₂ material according to the invention is suited forproducing an optical component, particularly for microlithography,including immersion lithography, on account of its high transparency inthe short-wave UV range, its UV radiation resistance and its chemicalresistance.

Particularly with respect to the use of the material of the inventionfor producing optical components for use with radiation within theshort-wave UV wavelength range, it has turned out to be useful when thequartz crystallites have a mean grain size in the range between 750 nmand 20 μm, preferably a mean grain size in the range between 1 μm and 15μm, and particularly preferably a mean grain size in the range between 2μm and 10 μm.

The upper and lower limits of these grain size ranges follow, accordingto the above-explained observations regarding the scattering amount dueto birefringence, the loss portion of the transmission due to scatteringon grain boundaries, the reduction of the effect of birefringence, fromthe statistic distribution of the crystallite orientation.

As for a low scattering, it has turned out to be advantageous when thematerial has a total porosity of less than 1 ppm.

In this context the size of possible pores in the material also plays acentral role. Advantageously, the size of existing pores is less than 1μm, preferably less than 0.5 μm.

As for the method for making a blank from the material, theabove-mentioned object is achieved according to the invention by amethod comprising the following measures:

-   -   (A) providing granules consisting of synthetically produced        quartz crystals having a mean grain size in the range between        500 nm and 30 μm, and    -   (B) sintering the granules to obtain a blank of polycrystalline        quartz.

For preparing a material for an optical component according to thepresent invention granules of synthetically produced quartz crystalshaving a specific grain size range are sintered.

-   -   The synthetic production of the quartz crystals achieves a high        purity of the material and thus an adequate transparency in the        wavelength range below 250 nm to about 150 nm.    -   During sintering of the quartz crystals to obtain a blank of        polycrystalline quartz the grain size thereof does not change or        varies only little. The sintered polycrystalline blank thus        consists of quartz crystallites having a mean grain size ranging        from 500 nm to 30 μm, whereby the effects described above with        reference to the material according to the invention and        regarding reduced light scattering and birefringence are        achieved.    -   The material prepared in this way is distinguished by reason of        its crystalline structure by a resistance to short-wave UV        radiation that is much higher in comparison with quartz glass.

The polycrystalline SiO₂ material produced according to the invention issuited by virtue of its high transparency in the short-wave UV range,its UV radiation resistance and its chemical resistance for theproduction of an optical component, particularly for microlithography,including immersion lithography.

In a preferred embodiment of the manufacturing method of the invention,providing the quartz crystal granules according to measure (A) comprisesthe following method steps:

(a) forming amorphous SiO₂ by hydrolysis or oxidation of a siliconcompound which is vaporizable in the temperature range of up to 500° C.,

(b) using the amorphous SiO₂ as a raw material for preparing syntheticquartz seed crystal, and

(c) using the quartz seed crystal for forming quartz crystal granules.

In a first method step, amorphous SiO₂ particles are formed by means ofthe known plasma or CVD deposition methods (OVD, VAD, MCVD PCVD, or thelike), wherein a synthetic silicon compound that is vaporizable up to500° C. is used as the glass start material and amorphous SiO₂ particlesare formed therefrom.

Thanks to the vaporizability of the synthetic silicon compound at atemperature below 500° C., which can still be handled easily, arelatively easy ultrapure preparation of the compound can be achieved.This has a particularly advantageous effect on the purity of theamorphous SiO₂ particles.

The particulate or massive SiO₂ raw material produced in this way isused for making synthetic quartz seed crystals. The amorphous SiO₂ rawmaterial is here transformed into crystalline material, the knownmethods for crystal synthesis by growing from the melt, growing fromsolutions and ceramic growing methods being suited. The syntheticallyproduced glass start material ensures a high purity both of theamorphous SiO₂ particles and the SiO₂ raw material produced therefrom,and also of the synthetic quartz seed crystal. The synthetic quartz seedcrystal produced in this way is present in single crystalline form, inpolycrystalline form or as a multitude of crystals (single-crystallineor polycrystalline).

The quartz seed crystal is directly used as quartz crystal granulationon condition that the grain size is directly suited, or it is processedinto the quartz crystal granules.

It has turned out to be particularly advantageous when the formation ofamorphous SiO₂ according to method step (a) comprises depositing SiO₂particles with formation of a massive preform of amorphous SiO₂.

This is a generally known and proven method step for making blanks ofsynthetic quartz glass for producing components for use in optics or intelecommunication engineering.

The massive glass-like preform of synthetic SiO₂ is subsequently used asa raw material for making synthetic quartz seed crystal.

This normally requires a mechanical or chemical breaking open of thepreform. Hence, the use of the amorphous SiO₂ as a raw material forproducing the synthetic quartz seed crystal according to method step (b)preferably comprises a breaking open of the preform of amorphous SiO₂,whereby the same is reduced in size or dissolved.

The preform can be broken open in a particularly easy manner when thepreform is formed from porous SiO₂.

Such preforms of porous SiO₂ are also designated as “soot bodies”. Theformation of the preform as a porous soot body permits or simplifiessubsequent processing, cleaning or doping, if desired. A dehydrationtreatment for reducing the hydroxyl group content should here beparticularly mentioned.

On the other hand, porous soot bodies tend to absorb substances from theenvironment, which makes their storing and handling difficult. It hastherefore turned out to be also useful when the preform is made fromtransparent or opaque quartz glass having a density of at least 2.1g/cm³.

The high density reduces the risk of contamination of the SiO₂ prior tothe use of the preform as a start material for crystal growing.

It has turned out to be useful when the production of the syntheticquartz seed crystal according to method step (b) is carried out by wayof a hydrothermal method.

In quartz crystal growing in the hydrothermal method according to theinvention, a solution of the amorphous SiO₂ is produced in the hotterarea of a pressure vessel, the solution being fed by the syntheticallyproduced amorphous SiO₂. In the colder area of the pressure vessel, oneor more crystallization seeds are arranged on which due to thetemperature gradient in the pressure vessel synthetic quartz crystalcrystallizes. A crystal pulling from the melt and the accompanying riskof contamination caused by the crucible material can thus be avoided. Itis also because of the high-purity start material that the resultingquartz seed crystal is distinguished by a particularly high purity.

As a rule, quartz seed crystal is obtained with a grain size that is toolarge for the intended use. In this case the processing of the quartzseed crystal into quartz crystal granules according to method step (c)preferably comprises a crushing of the quartz seed crystals.

The crushing of the quartz seed crystal is carried out in the simplestcase with mechanical means by grinding, shaking, ultrasound, or thelike, and the crushing effect may here be supported by thermal measures(quenching) or chemical means (etchants).

Particularly with respect to the use of the material of the inventionfor making optical components for use with radiation in the short-waveUV wavelength range it has turned out to be useful when syntheticallyproduced quartz crystals are provided with a mean grain size in therange between 750 nm and 20 μm, preferably with a mean grain size in therange between 1 μm and 15 μm, and particularly preferably with a meangrain size in the range between 2 μm and 10 μm.

In a particularly preferred variant of the method the sintering of thequartz crystal granules to obtain a blank according to measure (B)comprises gas pressure sintering.

During gas pressure sintering the quartz crystal granules to be sinteredare heated under increased pressure and heated in this process at atemperature below the melting temperature of quartz. The overpressureaccelerates the sintering process and reduces possible pore formation.This reproducibly yields a crystalline material which has a totalporosity of less than 1 ppm. The pore size of possible residual pores isless than 1 μm, preferably less than 0.5 μm.

It has turned out to be advantageous when the blank is treated for anoptical component, the treatment comprising a removal of edge portionsof the blank.

The edge portion of the blank may differ in its thermal properties andits chemical composition from the inner volume, which may lead toundesired elastic stresses in the blank. Preferably, so much edge volumeis removed from the blank that the polycrystalline blank is alreadypresent in an almost final dimension for the component to be produced.To this end, compared with the final dimension of the blank, the blankhas an allowance of at least 10% (based on the respective dimensionprior to the removal of the allowance). For the elimination of residualstresses, it is advantageous to anneal the blank, the annealing processbeing preferably carried out prior to the removal of the edge volume.

The invention will now be explained in more detail with reference to anembodiment. A lens for a projection system for immersionmicrolithography is here produced.

1. Preparing Granules from Synthetically Produced Quartz Crystals

1.1 Forming Amorphous SiO₂ by Flame Hydrolysis of SiCl₄

An SiO₂ soot body is produced according to the so-called OVD method, asis otherwise standard in the manufacture of quartz glass bodies fromsynthetic SiO₂. For this purpose SiO₂ particles are deposited layer bylayer on a carrier rotating about its longitudinal axis by reciprocatingan assembly of deposition burners. The deposition burners are here fedwith SiCl₄ as glass start material and said material is hydrolyzed in aburner flame in the presence of oxygen into SiO₂.

After completion of the deposition process and removal of the carrier ahollow cylindrical soot body is obtained which for removing the hydroxylgroups introduced due to the production process is subjected to adehydration treatment. To this end the soot tube is introduced invertical orientation into a dehydration furnace and is treated at atemperature in the range around 1200° C. in vacuum.

The treatment period is about three hours, resulting in a hydroxyl groupconcentration of about 20 wt ppm.

The SiO₂ soot body treated in this way is then vitrified in avitrification furnace at a temperature in the range of about 1,600° C.into a transparent quartz glass body and said body is subsequentlycomminuted in a mill having an inner lining of quartz glass, fragmentsbeing here obtained within a wide size range.

1.2 Insertion of the Quartz Glass Fragments for the Synthesis of SeedCrystals

The quartz glass fragments are used as raw material for making syntheticquartz crystals according to the “hydrothermal method”.

In a vertically oriented autoclave a pressure of 120 bar and atemperature gradient between 350° C. (upper portion) and 400° C. (lowerportion) is produced. In the lower portion the quartz glass fragmentsare dissolved in a slightly alkaline solution. In the upper portion ofthe autoclave quartz glass plates cut in oriented fashion are arrangedas seeds. Due to the temperature gradient from the bottom to the top thequartz glass dissolved in the lower portion will condense on the quartzplates with formation of a synthetic quartz seed crystal at a crystalgrowth rate of about 1.5 mm/day.

The quartz seed crystal prepared in this way is distinguished by aparticularly high purity. The following typical impurity contents aremeasured (data in brackets in wt ppb): Li (100), Na (15), K (<20), Mg(<20), Ca (<30), Fe (70), Cu (<10), Ti (<10), and Al (20).

1.3 Use of the Quartz Seed Crystal for Forming the Quartz CrystalGranules

The quartz seed crystal prepared in this way is ground in a mill havingan inner lining of quartz glass in a dry milling method to obtain a finepowder of synthetic quartz crystal. The fine portion having grain sizesbelow 100 nm is separated from the powder obtained. The resulting quartzgranules have a mean diameter of 5 μm and consist essentially of roundgrains.

2. Sintering the Quartz Crystal Granules to Obtain PolycrystallineQuartz

The resulting synthetic quartz crystal granules serve as start materialfor producing a blank from polycrystalline quartz by gas pressuresintering.

The synthetic granules are here put into a graphite mold and are treatedat a temperature of 1600° C. The graphite mold is first heated to asintering temperature of 1,600° C. while maintaining a negative pressureof less than 1 mbar. After the sintering temperature has been reached,an overpressure of 10 bar is set in the furnace and the mold is kept atsaid temperature for about 3 hours. Cooling to a temperature of 400° C.is subsequently carried out at a cooling rate of 2° C./min, theoverpressure being further maintained. Free cooling to the roomtemperature is then carried out.

This yields a homogeneous stress-free solid cylinder of polycrystallinetransparent quartz with a residual porosity of 0.5 ppm, the quartzcrystallites having a mean grain size of around 5 μm.

The solid cylinder has an outer diameter of 300 mm and a height of 80mm. It is treated to obtain a lens blank for a projection objective inthat an edge layer of 5 mm is ground off from the faces, and an edgelayer having a thickness of 20 mm from the outer cylindrical surface.

The projection objective is distinguished by high transparency in theshort-wave UV range, high UV radiation resistance and excellent chemicalresistance to almost all media and is therefore suited for use inmicrolithography, particularly as the last optical component withcontact with the immersion liquid in immersion lithography.

1. A material for an optical component, said material comprising:synthetically produced quartz crystallites that form a polycrystallinestructure and that have a mean grain size in a range between 500 nm and30 μm.
 2. The material according to claim 1, wherein the quartzcrystallites have a mean grain size in a range between 750 nm and 20 μm.3. The material according to claim 1, wherein said material has a totalporosity of less than 1 ppm.
 4. The material according to claim 1,wherein the the quartz crystallites have pores with a size less than 1μm.
 5. A method for making a material, for an optical component saidmethod comprising: providing granules of synthetically produced quartzcrystals having a mean grain size in a range between 500 nm and 30 μm,and sintering the granules to obtain a blank of polycrystalline quartz.6. The method according to claim 5, wherein the step of providing thegranules comprises: forming amorphous SiO₂ by hydrolysis or oxidation ofa silicon compound which is vaporizable in a temperature range of up to500° C., using the amorphous SiO₂ as a raw material for preparingsynthetic quartz seed crystal, and processing the quartz seed crystal soas to form quartz crystal granules.
 7. The method according to claim 6,wherein forming amorphous SiO₂ comprises depositing SiO₂ particles so asto form a massive preform of amorphous SiO₂.
 8. The method according toclaim 7, wherein the step of using the amorphous SiO₂ as the rawmaterial for preparing the synthetic quartz seed crystal comprisesbreaking open the preform of amorphous SiO₂.
 9. The method according toclaim 7, wherein the preform is formed from porous SiO₂.
 10. The methodaccording to claim 7, wherein the step of preparing the synthetic quartzseed crystals is carried out by a hydrothermal method.
 11. The methodaccording to claim 6, wherein processing the quartz seed crystals so asto form quartz crystal granules comprises crushing the quartz seedcrystals.
 12. The method according to claim 5, wherein the syntheticallyproduced quartz crystals have a mean grain size in a range between 750nm and 20 μm.
 13. The method according to claim 5, wherein the sinteringof the quartz crystal granules to obtain a blank comprises gas pressuresintering.
 14. The method according to claim 5, wherein a treatment isapplied the blank to obtain a blank for an optical component, thetreatment comprising removal of edge portions of the blank.
 15. Thematerial of claim 1, wherein the material is used for an opticalcomponent in microlithography.
 16. The material according to claim 1,wherein the quartz crystallites have a mean grain size in a rangebetween 1 μm and 15 μm.
 17. The material according to claim 1, whereinthe quartz crystallites have a mean grain size in a range between 2 μmand 10 μm.
 18. The material according to claim 1, wherein the quartzcrystallites have pores with a size less than 0.5 μm.
 19. The methodaccording to claim 5, wherein the synthetically produced quartz crystalshave a mean grain size in a range between 1 μm and 15 μm.
 20. The methodaccording to claim 5, wherein the synthetically produced quartz crystalshave a mean grain size in a range between 2 μm and 10 μm.